The present invention relates to a projection exposing apparatus used in manufacturing semiconductor devices and, more particularly, to an illumination system for a projection exposing apparatus having an improved fly's eyes lens.
FIG. 1 illustrates schematically a conventional illumination system for a projection exposing apparatus that is used in manufacturing semiconductor devices. The system includes a reflection mirror 2 placed in front of a light source 1, a fly's eyes lens 3 and a condenser lens sequentially located to the rear of light source 1. The fly's eyes lens 3 contains many small lenses each of which has a diameter smaller than several millimeters.
In such an illumination system, the light from light source 1, incident to fly's eyes lens 3, varies widely in intensity according to location. After the light passes through fly's eyes lens 3, more uniform illumination is achieved on an image-forming surface by the plurality of fly's eye lenses.
Recent developments in exposure technology demand finer resolution, therefore, the exposure area must become smaller or the number of lenses must be reduced. In practice, this is difficult to accomplish and still obtain uniform illumination. Phase shift techniques or annular illumination techniques, which have been recently developed and employed in the field in order to perform a high resolution tilted illumination, block the central light passing the fly's eyes lens using a filter and only 20-50% of the light is used. This deteriorates the illumination uniformity. Further, even if the number of lenses can be increased by reducing the size of individual lenses, because the fly's eyes lens consists of 50-100 individual lenses, the overall manufacturing cost is expensive and manufacturability of the required small lenses is too difficult to be practical.
An alternative to the conventional fly's eyes lens uses a zone plate, which is a lens that diffracts light. As shown in FIGS. 2A and 2B, the zone plate forms a concentric circular pattern so that light is alternately transmitted or blocked. This pattern can reduce the size of individual lenses and form a number of lenses, to thereby improve the illumination uniformity. However, since light is blocked in one-half of the lenses, the overall light transmission efficiency decreases to below 50%. Thus, the zone plate has not been widely employed despite its simple manufacturability.